Due to their ease of operation, cutting speed, lightweight and high versatility, the portable powered chain saw has today virtually replaced the one and two man blade saws previously used for felling and trimming trees in the lumber and logging industries. Likewise, in the private consumer market, from the professional and occasional tree trimmer to the homeowner cutting his own fireplace or furnace wood, the chain saw has become a modern day necessity. Such wide spread and versatile use demands have emphasized more than ever the need for chain saws with improved reliability, safety, versatility and efficiency in operation.
Depending upon the size of the wood severing operation to be performed by the chain saw, its size and power rating will vary. However, such chain saws customarily include a lightweight driving motor, typically a small gasoline powered engine, an elongated guide bar extending in cantilevered manner out from the motor, and an endless articulated chain carrying spaced cutting members thereon which serve as the cutting blade for the saw. The guide bar and chain are cooperatively designed such that the chain moves or tracks along the periphery of the guide bar and is looped over a sprocket aligned at the motor end of the guide bar, which sprocket is driven by the motor. When the motor is operated so as to drive the sprocket, the sprocket pulls the endless cutting chain along the periphery of the guide bar, moving the cutting members therealong. A cutting or sawing operation is performed by positioning the guide bar in proximity with an object such that the moving cutting members engage the object at the desired "cut" position, thus severing upon contact therewith small particles from the object.
The theory of operation of such chain saws is very simple. However, due to the fact that in operation, the cutting chain is constantly moving in frictional engagement with the underlying guide bar, chain saws have historically been very difficult to keep operating at maximum efficiency for any extended usage without requiring frequent re-adjustment in the field. Such re-adjustment, besides being burdensome on the operator, reduces the time that would otherwise be devoted to cutting operations, often requires the operator to carry an adjustment tool kit with him, and leaves entirely to the judgment of the operator the decisions as to when, in what manner, and to what extent, such adjustments will be made. Failure to make timely or proper adjustments can result in a safety hazard to the operator with further operation of the saw, as well as reducing the efficiency, reliability, and longevity of the chain saw and its component parts.
The primary parameter responsible for the adjustment problem is the tension of the cutting chain relative to the guide bar. The tension must be sufficiently "tight", such that the chain will stay within the peripheral guide track of the guide bar. Obviously, if the chain tension is too loose, the chain can jump out of the guide bar track, causing a dangerous situation to the operator. A loose saw chain will typically continue to "travel" within the guide track even when the drive sprocket is not being driven. This can create a very dangerous condition to the operator of the saw, or to onlookers. It has also been found that a loose chain will "slap" the guide bar to such an extent during operation of the saw, that it will actually flange or roughen the engaging surfaces of both the chain and the guide bar, thus requiring more power from the drive motor to overcome the increased friction. If the chain tension is set too tightly, the frictional forces between the chain and the guide bar will cause excessive early wear on the chain and the guide bar as well as causing over-heating of the cutting chain and can cause the chain to bind in the guide bar, resulting in a dangerous situation to the operator should the chain break as a result thereof. Such over-heating of the chain also results in loss of temper in its cutting teeth, necessitating frequent filing or sharpening of the teeth by the operator.
While the necessity of proper chain saw tension has long been recognized in the art, the ability to maintain the desired uniformity of such tension over extended periods of operative use, has not been realized. While a chain tension may be properly set prior to use of the chain saw, the tension will change as the saw is operated over a period of time. Many factors contribute to the change. One of the primary factors affecting such change is the difference in the temperature coefficients of expansion between the cutting chain and the guide bar materials. As the guide bar and cutting chain heat up during cutting operations, the chain material typically expands faster than that of the guide bar, causing the chain tension to slacken. The result can cause a snow-balling effect (i.e. the decreased chain tension causes even greater frictional drag forces on the chain during the cutting operations due to normal operation and due to the chain and guide bar deterioration that results from chain "slap", which further increases the temperature, and contributes even more to the decrease of tension). Other factors such as the sharpness and alignment of the chain cutting members, the environment (i.e., wet snow, dry, etc.) in which the saw is being used, the type, consistency and nature of the wood or other object being cut, the proper oiling of the chain, use and misuse by the operator, and the like--all contribute to the problem of maintaining proper cutting chain tension in operative use.
Thus, in order to prevent the chain from loosening and from possibly jumping out of the guide bar track, the operator must interrupt his cutting operations to reset the tension of the expanded chain. An impatient operator may try to minimize the number of times he should re-adjust the chain tension, by overtightening the chain, resulting in a dangerous chain binding situation. Obviously, should the operator ever take a rest of sufficient length to enable the chain to cool, its length will shorten during the rest interval, rendering the chain tension too tight upon resumption of cutting operations. Similarly, even under continuous cutting operations, should the conditions under which the saw is being used abruptly change, (i.e., such as a change in the type or consistency of the wood being cut) so will the temperature effect upon the cutting chain--again requiring resetting of the chain tension.
Heretofore, attempts have been made in the prior art, to address the tensioning problem. None of such attempts, however, have resulted in devices which eliminate the periodic tensioning adjustment of the saw chain or which are economically practical, and adapted to the rugged and varied uses to which chain saws are typically put. For example, the prior art recognizes the advantage of altering the guide bar structure to place a free-wheeling sprocket or pulley at the distal (nose) end of the guide bar, thus reducing the frictional drag of the chain against the guide bar at its distal end. U.S. Pat. No. 3,279,508 to Ehlan et al illustrates a variation of this concept.
A number of patents have dealt specifically with providing simplified means for performing the tensioning adjustment procedure in the field. See for example, U.S. Pat. Nos. 2,765,821 to Strunk, 3,327,741 to Merz, and 3,267,973 to Beard. Each of these patents illustrates a tensioning mechanism whereby once the proper tension is set, the guide bar is rigidly secured to the primary chain saw chassis until subsequently manually re-adjusted.
Attempts have been made in the art to provide continuous automatic tensioning adjustment to the cutting chain. See for example, U.S. Pat. Nos. 2,316,997 and 2,532,981 to Smith and Wolfe, respectively. Both of the structures illustrated by these patents employ a bifurcated guide bar wherein the rearward portion of the guide bar is rigidly secured to the primary chain saw chassis, and the distal end of the bifurcated bar is resiliently mounted under spring tension into engagement with the cutting chain, to adjust for tension variations in the cutting chain. While the basic theory behind these configurations is sound, neither of the structures illustrated was refined to the point of being commercially economical or operatively practical for use in the rugged environments in which chain saws are typically used. One particular shortcoming of these structures is their exposure of critical elements to damaging external environments. The exposed parts are inherently susceptible to moisture deterioration and seizing (due to rust) as well as to physical damage and degradation.
More recent developments in the art have abandoned the bifurcated guide bar approach in favor of configurations which apply tension adjusting forces to a single guide bar that is reciprocally mounted to the primary chain saw chassis. See for example U.S. Pat. Nos. 3,194,284 and 3,636,995 to Walker and Newman, respectively. Reliability and accuracy of such tensioning structures, however, is severely strained by the transmittal of large leverage forces thereto through the elongate guide bar. Such structures also typically display poor transfer of lubricating oil from the oil reservoir on the drive unit, to the moving chain. Further, the resilient mounting of the guide bar in such structures does little to minimize and may enhance vibratory forces inherently present in the chain saw operation. A further shortcoming of such structures is that they are typically peculiar to the particular chain saw frame or chassis used, and do not lend themselves universally applicable to chain saw guide bars that can be used with the existing chassis configurations of a number of different manufacturers.
The present invention comprises a composite structurce which ovcrcomes, in one device, most of the collective shortcomings of the prior art tensioning structures. The guide bar and tensioning structure of the present invention maintain a constant, uniform tension on the cutting chain. The guide bar and tensioning structure of the present invention are simple, properties that significantly reduce the operative vibration typically found in pior art chain saws. Chain and guide bar wear are significanlty reduced, thus increasing their operative lives. With the maintenance of proper tensioning provided by the inventive structure, the motor/engine efficiency of the saw is significantly increased, since more of the drive power is available for the task of cutting, rather than being spent in overcoming frictional and mis-alignment forces heretofore present in the cutting operation. Fuel consumption of the saw is accordingly reduced, for a given cutting task, and operator efficiency is increased due to the elimination of non-productive time heretofore required to periodically adjust the cutting chain tension and to prematurely resharpen the cutting teeth of the chain. Operator fatigue is reduced due to the lower vibration levels displayed by the chain saw, and cumbersome adjustment tool kits and lubricating grease guns are eliminated with the present invention. The structure of the present invention can be universally adapted to fit the saw chassis configurations of most chain saw manufacturers currently in the field. Critical moving parts are shielded from damaging external environments, while improved lubrication techniques significantly enhance their operation, reduce wear and increase reliability.